1. Field of the Invention
This invention relates to methods and apparatus used to form threads in machine components--especially those in which threads must be formed in a partial, often a half, hole which intersects the peripheral edge of a component.
2. Description of the Prior Art
In the manufacture of machine components such as sprockets it is common to use bushings of differing sizes to minimize the number of sprockets required to accommodate the large variation in shaft sizes. This enables manufacturing efficiencies which would not be otherwise achieved.
One popular method for reducing manufacturing costs involves the compression of powdered metals in a die to accurate dimensions prior to sintering to the final metallurigcal consistency. The bushing used to adapt a sprocket to its supporting shaft is an example of a product that may be economically manufactured using powdered metal, as exemplified in U.S. Pat. No. 4,525,094, "Powdered Metal, Flanged Bushing for Securing Machine Elements to Shafts", June 25, 1985.
An earlier example of a bushing of commonly used configuration may be seen with reference to U.S. Pat. No. 3,677,583, "Bushing and Method of Manufacturing", July 18, 1972. Here, the bushings are tapered for use with hubs, each having a tapered internal wall corresponding to an external taper on an associated bushing. Screws are used in threaded holes formed between and intersecting the mating tapers to retain the bushing. This particular method of manufacture includes the steps of placing bushing blanks having the tapered surfaces together and drilling and tapping a single hole in the area of contact between them. Additional slots or grooves are formed on the periphery of each bushing blank, usually extending about one half the distance from one end of the blank to the other. These slots are also commonly formed by drilling, requiring an additional operation that adds to manufacturing costs. An improved method and apparatus for manufacturing powdered metal bushings and for forming the slots is disclosed in my copending patent application, "Method of Manufacturing Bushings with Pwodered Metals", Ser. No. 122,839, filed Nov. 19, 1987.
More importantly to this invention, the '583 patent discloses a method of fabricating tapered bushing for use with hubs that have a tapered internal wall corresponding to the external taper on the bushings. Each hub and tapered bushing is to be ultimately connected during the assembly operation with screws inserted in holes intersecting the tapered surfaces. The bushing manufacturing method includes the step of forming two essentially identical metal blanks, each having tapered external surfaces with a groove in the surface parallel with the axis of the blank. Then the two blanks are placed side-by-side with mating flat surfaces of their peripheries in contact and the grooves aligned to form a round hole, one half being in each bushing. Finally, the round hole is threaded to form a threaded half hole in each blank or bushing.